1. Field of the Invention
The invention relates to a variable valve timing apparatus for an internal combustion engine and a control method thereof.
2. Description of Related Art
A conventional variable valve timing apparatus for an internal combustion engine includes a variable valve timing mechanism that varies valve characteristics of engine valves such as an intake valve and an exhaust valve, an actuator that is driven within a predetermined driving range in order to activate the variable valve timing mechanism, and an electronic control apparatus for drive-controlling the actuator.
To control the valve characteristics of the engine valves with a high degree of precision in this type of variable valve timing apparatus, it is important to accurately detect actual valve characteristics and activate the variable valve timing mechanism, or in other words to drive-control the actuator, such that the actual valve characteristics are matched with target characteristics. As regards a method of detecting the actual valve characteristics of the engine valves, considering that the valve characteristics of the engine valves correspond to a driving position of the actuator within the aforesaid driving range, a position sensor that detects the driving position of the actuator may be provided such that the actual valve characteristics of the engine valves are detected using the driving position of the actuator detected by the position sensor. Note that the driving position of the actuator detected by the position sensor (more accurately, information relating to the driving position) is stored in a random access memory (RAM) of the electronic control apparatus. Thus, the information relating to the driving position of the actuator stored in the RAM of the electronic control apparatus is read from the RAM when required, for example to detect the actual valve characteristics of the engine valves.
However, the information relating to the driving position of the actuator used to detect the actual valve characteristics of the engine valves does not always correspond to the actual driving position of the actuator and may deviate from the actual driving position. When noise is generated in a signal from the position sensor, for example, the actuator driving position detected by the sensor, or in other words the information relating to the driving position stored in the RAM of the electronic control apparatus, may take an inaccurate value. Further, the information relating to the driving position stored in the RAM of the electronic control apparatus may be reset to an initial value and thereby lost, or the content of the information may change, when a power supply to the electronic control apparatus is temporarily stopped (when a so-called instantaneous power cut occurs) or the like. In these cases, the actuator driving position detected by the position sensor, or in other words the information relating to the actuator driving position stored in the RAM of the electronic control apparatus, becomes inaccurate. As a result, the valve characteristics of the engine valves detected on the basis of the driving position information also become inaccurate. Hence, when the actuator is driven on the basis of the detected valve characteristics in an attempt to control the valve characteristics of the engine valves to the target characteristics, the control cannot be executed correctly.
In response to this problem, an initialization process is executed to match the actuator driving position detected by the position sensor with an actual actuator driving position when a predetermined execution condition is established. More specifically, the initialization process is performed through [Procedure 1] to [Procedure 3] described below.
[Procedure 1]
The actuator driving position detected by the position sensor, or in other words the information relating to the driving position stored in the RAM of the electronic control apparatus, is set at an initial value in a state where the actuator is driven to one end of the driving range.
[Procedure 2]
The actuator is driven to an opposite end of the driving range that is opposite to the one end of the driving range, and in this state, an offset amount from an appropriate value of the actuator driving position detected by the position sensor is obtained.
[Procedure 3]
The offset amount from the appropriate value of the detected actuator driving position is reflected in the driving position to compensate for the offset amount, and a reflected value is stored in the RAM of the electronic control apparatus as corrected information relating to the driving position.
Note that Japanese Patent Application Publication No. 2009-216052 (JP-A-2009-216052) describes driving the actuator from one end of the driving range to the opposite end of the driving range when executing the process for matching the actuator driving position detected by the position sensor with the actual driving position.
By executing the initialization process described above, the actuator driving position detected by the position sensor can be matched with the actual actuator driving position with a high degree of precision. The reason why the detected driving position can be matched with the actual driving position with a high degree of precision is that the information relating to the actuator driving position is set at the initial value at one end of the actuator driving range, whereupon the offset between the information relating to the actuator driving position and the actual driving position is corrected at the opposite end of the driving range that is opposite to the one end of the driving range. In other words, during the initialization process, as illustrated by [Procedure 1] to [Procedure 3], the actuator is driven (caused to perform a full stroke) from one end of the actuator driving range to the opposite end of the driving range that is opposite to the one end of the driving range.
However, when the actuator is driven by a full stroke in the initialization process described above, the valve characteristics of the engine valves invariably undergo great variation due to the accompanying operation of the variable valve timing mechanism, and this variation greatly affects an engine operation. Hence, the initialization process can only be executed during special engine operations when the aforementioned effect does not have to be taken into account, for example when the internal combustion engine is operated for the first time following recovery from an abnormality (an abnormality recovery operation) or the like, and it is therefore difficult to perform the initialization process during a normal engine operation. When the execution frequency of the initialization process decreases in this manner, the information relating to the actuator driving position stored in the RAM of the electronic control apparatus remains at an offset value from the actual driving position, and therefore, in many cases, the actuator is drive-controlled continuously using this inaccurate information. When the actuator is drive-controlled using the inaccurate information, the valve characteristics of the engine valves become inappropriate for the engine operation. As a result, a combustion condition of the internal combustion engine may deteriorate, leading to a reduction in an operability of the engine and an increase in exhaust emissions.